1. Field of the Invention
The present invention relates generally to radiation sensitive solid state device and, in one of its aspects, to a photodiode which utilizes a lateral current through an N region beneath a radiation sensitive P-type element.
2. Description of the Prior Art
Typically, a photodiode comprises a N-type semiconductor layer and a light sensitive P-type element adjacent a first surface and forming a rectifying junction with the N-type layer. There is a connection of the N-type layer and a connection of the light sensitive P-type element. The photodiode is normally back biased in operation with the N-type region at a higher voltage potential than the P-type element. With no light incident upon the photodiode, only the dark leakage current flows from the connection of the N-type layer to the connection of the light sensitive P-type element. When light shines on the photodiode, typically on the P-type element, photocurrent passes through the photodiode from the connection of the N-type layer to the connection of the light sensitive P-type element. This photocurrent is normally far greater than the dark leakage current.
In most switching applications of photodiodes and phototransistors, the turn-off time is extended due to the long lifetime of the photogenerated carriers and the relatively slow diffusion velocity of these carriers. In situations where the P-type element is more heavily doped than the N-type layer, excess carriers are present throughout the N-type side of the diode when turning off and, therefore, some carriers must diffuse the entire distance across the N-type layer.